(1) Field of the Invention
This invention relates to fin-stabilized ammunition, and more particularly to armor-piercing fin-stabilized discarding sabot with tracer (APFSDS-T) ammunition.
(2) Description of the Related Art
There exists a well-developed art in the field of APFSDS (including, inter alia, APFSDS-T (with tracer)) ammunition. APFSDS rounds have been developed for both rifled and smoothbore barrels (tubes). A rifled barrel or tube functions to spin-stabilize a projectile encased in the sabot, a principle utilized in weapons from handguns to large naval guns. A projectile exiting the muzzle of a rifled tube typically has a relatively high spin rate. This rifling-induced spin rate is nominally equal to the product of the muzzle velocity (longitudinal) and the rifling pitch (measured in turns or revolutions per linear dimension) at the muzzle. An exemplary 105 mm rifled tube has a 1-18 twist, meaning the longitudinal distance the rifling travels downbore to make one complete revolution is eighteen times the caliber of the barrel. Thus, the exemplary pitch is one turn per 1.89 meters. With an exemplary muzzle velocity of from about 1,375 to about 1,650 meters per second, the associated spin rate will be from about 730 to about 870 revolutions per second (rps). Such a spin rate would adversely affect the performance of an APFSDS round as, once the projectile (also occasionally designated “sub-projectile”) is free of the sabot, it relies on its aerodynamic fins for stability at a relatively low spin rate. The rapid angular deceleration from the rifling-induced spin rate to the preferred low spin rate may: (a) damage the sub-projectile; (b) require a weight penalty associated with providing particularly robust fins to avoid damage; and/or (c) induce wobble or other forms of instability.
Common APFSDS rounds for rifled tubes decouple rotation of the projectile from rotation induced by the rifling by providing the sabot with a “slip obturator” mounted on the sabot body in such a way as to allow the obturator to rotate about the longitudinal axis of the sabot. The obturator engages the tube bore, accommodating to the rifling and forming a seal to retain propellant gases behind the obturator. Because of its accommodation to the rifling, the obturator acquires the rifling-induced spin rate described above. With a slip obturator, this spin rate, however, is not entirely translated to the combination of the sabot body and sub-projectile. The sabot/projectile combination typically has sufficient moment of inertia about the longitudinal axis to overcome the static frictional force along the annular engagement between the obturator and sabot body to allow rotation of the obturator relative to the sabot body. Thus, the sabot body and projectile spin at a rate less than the obturator. A properly designed slip obturator results in a projectile spin rate which is a small percentage of the rifling-induced spin rate. Once the projectile is free of the sabot, it relies on its aerodynamic fins for stability as a means to maintain relatively low spin rate (e.g., about 70 revolutions per second (rps)). This is accomplished by the torque applied to the sub-projectile created by the aerodynamic force of airflow over its fin blades. Typically fin blades are chamfered, canted, or deflected in such a manner that the airflow over the projectile creates a rotational force on any forward-facing projected areas of the fin blades. Commonly APFSDS rounds are spun at low rates to normalize any physical imbalances and/or aerodynamic forces that they would be subjected to while in flight to the target.
In the subfield of medium caliber ammunition (e.g., nominal caliber 20 mm to 60 mm), APFSDS-T ammunition has also been developed. A key example is the 25 mm M919 round used by the 25 mm M242 Bushmaster automatic gun of the U.S. Army M2/M3 Bradley Fighting Vehicle (BFV). When fired from the 2.003 m long barrel of the M242, the M919 projectile has an exemplary muzzle velocity of about 1385 m/s at ambient conditions and, more broadly between about 1345 m/s and about 1400 m/s at the round's required temperature extremes. The projectile has an effective range in excess of 1500 m. With a gain twist barrel of a groove-to-groove diameter of 26.0 mm, a land-to-land diameter of 25.0-25.1 mm and a 7.5 degree twist at the muzzle, a full spin projectile would leave the muzzle with a theoretical spin rate of about 2320 rps at the exemplary 1385 m/s muzzle velocity. The effect of a slip obturator is to substantially reduce the muzzle spin rate of the projectile (e.g., to about 15-50 percent, or more particularly about 25 percent of the theoretical value).
After exiting the barrel and discarding the sabot, the projectile spin rate decays further, initially quite sharply and then more gradually, eventually approximating a steady state condition (see, U.S. Pat. No. 4,815,682 of Feldmann et al.). For consistency herein, where a numerical value is given, the steady state spin rate (SSSR) is defined as the spin rate of the projectile one second after exiting the muzzle when fired at a minimal angle of elevation under standard conditions.
Tracer visibility is critical to a weapon operator as it should allow the operator to follow the projectile flight path to permit the operator to re-aim the weapon to hit a desired target. In such small size, high velocity, long range rounds as the M919, tracer visibility has been a significant problem (see, U.S. Pat. No. 5,472,536 of Doris et al. which discloses improved visibility tracer compositions). An operator, utilizing the sighting systems of the weapon firing such a round may have a hard time acquiring the tracer in the sights and maintaining sight of the tracer.